In anisotropic high-rate etching of silicon substrates, for example in the manner referred to in German patent publication no. 42 41 045, it is necessary to cool the substrate from its back, since significant quantities of heat are brought into the substrate from the plasma through the effect of rays, electrons and ions, as well as through heat of reaction developing on the wafer surface. If this heat is not dissipated in a controlled manner, the substrate overheats and the etching result worsens significantly.
U.S. Pat. Nos. 6,267,839 and 5,671,116, European patent document no. 840 434, and Japanese patent document no. 11330056, refer to so-called electrostatic “chucks,” i.e., holding devices via which a semiconductor wafer, in particular a silicon wafer, is electrostatically fixable on a substrate electrode, for example in a plasma etching device. Another holding device in the form of an electrostatic “chuck” is shown in FIG. 1. This configuration may be found today in plasma etching systems.
In detail, it is provided that the substrate electrode, to which for example a high-frequency voltage is applied, is clamped onto a grounded base plate by ceramic insulators and a suitable clamping device, O-rings ensuring the vacuum seal, so that the substrate to be etched is able to be subjected to a vacuum. It is also provided for the substrate electrode to have a cooling agent, for example deionized water, methanol or other alcohols, fluorocarbons or silicones, flowing through it internally. Located on the substrate electrode itself is the “chuck” for electrostatically clamping the wafer or substrate lying on it, which is supplied with high voltage via conventional high-voltage feed-throughs, in order to exert the desired clamping force on the wafer positioned on top of it.
Finally, FIG. 1 provides that the surface of the “chuck,” not covered by the substrate, and the surrounding substrate electrode surface are covered by a ceramic plate placed on the substrate electrode, in order to prevent the plasma existing or produced above it from acting on the metal surfaces of the substrate electrode, which could result in detrimental sputtering-off of metal and an unwanted current flow into the plasma.
The heat flow from the underside of the overlaid wafer to the electrostatic “chuck” or the substrate electrode is guaranteed finally by a helium cushion, i.e., there are suitably shaped spaces between the underside of the wafer and the “chuck” and between the “chuck” and the substrate electrode surface, which are filled with helium at a pressure ranging from a few mbar to a maximum of about 20 mbar.
Alternatively to electrostatic clamping or fixing of a wafer, mechanical clamping devices are also known in the related art, which press the wafer onto the substrate electrode and allow helium to be applied to the back of the wafer as a convection medium. The mechanical clamping has substantial disadvantages, however, and is being replaced increasingly by electrostatic “chucks,” which particularly ensure favorable flat wafer clamping.